Seddon's Corner
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seddon's Corner clear sky chart from 2005-11-06 to 2024-10-20.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	12% 9% 4 9% 6% 5% 5% 4 6% 7% 33%	101744
	Night	10% 11% 4 9% 6% 5% 5% 4 6% 7% 32%	97061
Transparency	Day	48% 7% 15% 22% 7%	101485
	Night	50% 7% 14% 21% 8% 1	96967
Seeing	Day	15% 10% 30% 28% 13% 3	16214
	Night	16% 6% 31% 33% 13% 1	26091

By Month

Percentage Cloud Cover NW (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	7% 16% 6% 14% 8% 7% 8% 6% 6% 5% 18%	2219
	11th to 20th	Day	5% 9% 6% 18% 12% 12% 9% 6% 7% 4 11%	2199
	21st to end	Day	7% 20% 8% 17% 8% 5% 6% 5% 6% 5% 13%	2702
February	1st to 10th	Day	8% 13% 6% 11% 8% 7% 9% 6% 7% 8% 19%	1662
	11th to 20th	Day	8% 11% 4 10% 8% 9% 6% 6% 8% 8% 24%	1664
	21st to end	Day	6% 13% 6% 10% 9% 8% 7% 5% 5% 7% 24%	1446
March	1st to 10th	Day	9% 12% 4 12% 10% 5% 6% 5% 5% 6% 26%	3251
	11th to 20th	Day	7% 10% 6% 15% 12% 7% 6% 5% 5% 6% 23%	3478
	21st to end	Day	9% 15% 6% 12% 6% 4 4 3 4 6% 32%	3814
April	1st to 10th	Day	11% 11% 4 9% 6% 3 4 4 5% 6% 36%	2308
	11th to 20th	Day	13% 10% 4 7% 5% 5% 4 4 6% 7% 35%	2442
	21st to end	Day	14% 9% 3 7% 5% 4 5% 4 6% 7% 36%	2470
May	1st to 10th	Day	17% 9% 3 6% 4 4 4 3 5% 7% 38%	4334
	11th to 20th	Day	21% 8% 3 7% 5% 4 4 3 5% 7% 32%	4550
	21st to end	Day	21% 9% 3 7% 4 4 4 3 5% 6% 34%	5173
June	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 5% 8% 35%	2850
	11th to 20th	Day	17% 7% 3 7% 5% 5% 6% 4 5% 8% 34%	2850
	21st to end	Day	12% 7% 2 7% 6% 5% 6% 5% 7% 8% 33%	2850
July	1st to 10th	Day	10% 5% 2 6% 5% 6% 6% 5% 7% 9% 40%	4750
	11th to 20th	Day	8% 5% 2 4 3 4 4 4 6% 9% 50%	4750
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 49%	5150
August	1st to 10th	Day	8% 6% 2 7% 5% 5% 4 4 6% 9% 44%	4248
	11th to 20th	Day	11% 5% 2 6% 4 5% 4 4 6% 8% 43%	4041
	21st to end	Day	15% 6% 2 6% 4 3 5% 4 5% 9% 42%	4532
September	1st to 10th	Day	12% 5% 2 6% 5% 4 4 4 6% 7% 46%	2353
	11th to 20th	Day	15% 7% 2 7% 5% 4 5% 4 6% 9% 35%	2114
	21st to end	Day	18% 7% 3 6% 4 4 4 3 5% 7% 40%	2090
October	1st to 10th	Day	17% 12% 4 9% 4 4 4 3 4 5% 33%	1905
	11th to 20th	Day	14% 14% 4 9% 5% 4 5% 3 6% 6% 31%	1890
	21st to end	Day	18% 16% 5% 9% 6% 5% 5% 4 5% 7% 22%	1765
November	1st to 10th	Day	16% 17% 4 10% 4 6% 4 3 5% 6% 25%	1487
	11th to 20th	Day	10% 16% 5% 15% 8% 5% 4 3 5% 5% 22%	1520
	21st to end	Day	13% 14% 5% 15% 11% 5% 3 4 5% 6% 20%	1436
December	1st to 10th	Day	9% 14% 6% 18% 9% 8% 5% 5% 5% 6% 14%	1330
	11th to 20th	Day	10% 16% 6% 14% 6% 5% 5% 4 7% 8% 20%	1679
	21st to end	Day	10% 13% 5% 12% 6% 7% 6% 6% 8% 8% 19%	2442
January	1st to 10th	Night	5% 15% 7% 15% 8% 10% 8% 5% 5% 6% 16%	4737
	11th to 20th	Night	6% 14% 5% 15% 13% 13% 9% 5% 6% 5% 12%	4709
	21st to end	Night	8% 20% 8% 13% 7% 6% 6% 4 5% 5% 17%	4960
February	1st to 10th	Night	7% 13% 6% 11% 7% 6% 7% 5% 6% 7% 25%	2516
	11th to 20th	Night	6% 10% 4 9% 6% 7% 6% 6% 7% 7% 32%	2403
	21st to end	Night	6% 11% 6% 10% 7% 7% 6% 4 6% 6% 31%	2026
March	1st to 10th	Night	9% 12% 4 12% 7% 5% 6% 4 5% 7% 28%	3608
	11th to 20th	Night	5% 10% 4 9% 5% 5% 4 4 6% 8% 41%	3480
	21st to end	Night	11% 15% 4 9% 5% 4 4 3 6% 6% 33%	3817
April	1st to 10th	Night	11% 9% 3 5% 5% 4 5% 4 6% 7% 40%	1869
	11th to 20th	Night	14% 10% 4 8% 4 4 4 3 4 6% 38%	1689
	21st to end	Night	11% 10% 4 7% 5% 5% 6% 3 5% 5% 38%	1710
May	1st to 10th	Night	19% 9% 3 6% 4 3 4 3 5% 7% 37%	2634
	11th to 20th	Night	17% 7% 2 5% 4 5% 3 3 6% 7% 40%	2341
	21st to end	Night	18% 10% 2 4 4 3 3 3 6% 5% 40%	2414
June	1st to 10th	Night	12% 6% 2 6% 5% 4 5% 4 7% 10% 39%	1330
	11th to 20th	Night	16% 6% 2 5% 4 4 4 4 5% 9% 41%	1330
	21st to end	Night	12% 6% 2 5% 4 5% 4 4 6% 8% 44%	1330
July	1st to 10th	Night	9% 4 1 4 4 4 5% 4 6% 9% 50%	2220
	11th to 20th	Night	5% 3 1 4 3 4 4 4 6% 10% 54%	2220
	21st to end	Night	5% 3 2 4 4 4 5% 4 7% 10% 53%	2414
August	1st to 10th	Night	6% 4 2 5% 4 5% 5% 4 8% 10% 47%	2688
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 6% 9% 50%	2798
	21st to end	Night	11% 5% 2 5% 5% 4 5% 4 5% 9% 45%	3135
September	1st to 10th	Night	11% 4 2 6% 4 4 4 3 5% 7% 51%	1826
	11th to 20th	Night	16% 7% 2 7% 3 4 5% 3 4 8% 42%	2063
	21st to end	Night	17% 6% 3 6% 4 4 4 3 5% 7% 40%	2090
October	1st to 10th	Night	18% 9% 2 6% 5% 4 4 3 5% 8% 36%	2256
	11th to 20th	Night	17% 12% 3 9% 5% 4 4 4 5% 6% 32%	2275
	21st to end	Night	16% 15% 4 11% 5% 4 5% 3 5% 6% 27%	2589
November	1st to 10th	Night	14% 15% 5% 10% 6% 5% 5% 4 5% 5% 27%	2591
	11th to 20th	Night	11% 18% 6% 13% 7% 5% 4 3 4 6% 23%	2660
	21st to end	Night	12% 15% 6% 14% 9% 7% 6% 3 4 5% 19%	2740
December	1st to 10th	Night	8% 14% 5% 16% 10% 7% 6% 5% 6% 5% 18%	2850
	11th to 20th	Night	9% 15% 6% 14% 7% 5% 5% 5% 7% 7% 20%	3518
	21st to end	Night	8% 11% 5% 13% 8% 6% 7% 6% 7% 6% 22%	5225

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	65% 8% 11% 13% 4	2219
	11th to 20th	Day	70% 7% 12% 9% 2	2220
	21st to end	Day	70% 6% 10% 10% 4	2702
February	1st to 10th	Day	59% 5% 14% 13% 8% 1	1662
	11th to 20th	Day	53% 6% 14% 19% 7%	1673
	21st to end	Day	57% 7% 12% 14% 10%	1446
March	1st to 10th	Day	56% 6% 15% 18% 5%	3251
	11th to 20th	Day	55% 10% 13% 16% 6%	3480
	21st to end	Day	53% 6% 12% 22% 6% 1	3814
April	1st to 10th	Day	48% 6% 11% 23% 12% 1	2308
	11th to 20th	Day	48% 6% 11% 22% 12% 2	2442
	21st to end	Day	47% 6% 14% 23% 9% 1	2470
May	1st to 10th	Day	44% 7% 12% 25% 11% 1	4334
	11th to 20th	Day	49% 7% 10% 21% 11% 1	4554
	21st to end	Day	50% 7% 10% 23% 10% 1	5173
June	1st to 10th	Day	46% 7% 14% 22% 11% 1	2850
	11th to 20th	Day	47% 7% 14% 21% 10% 1	2850
	21st to end	Day	45% 9% 17% 21% 7% 1	2850
July	1st to 10th	Day	38% 9% 20% 26% 7%	4676
	11th to 20th	Day	29% 7% 20% 37% 7%	4669
	21st to end	Day	32% 7% 19% 34% 8%	4988
August	1st to 10th	Day	36% 7% 17% 31% 9%	4231
	11th to 20th	Day	36% 7% 21% 30% 5%	4080
	21st to end	Day	40% 6% 21% 28% 5%	4531
September	1st to 10th	Day	36% 8% 17% 29% 9% 1	2353
	11th to 20th	Day	45% 8% 19% 24% 4	2114
	21st to end	Day	45% 6% 13% 30% 7%	2090
October	1st to 10th	Day	54% 6% 15% 19% 6%	1905
	11th to 20th	Day	53% 6% 14% 21% 6%	1890
	21st to end	Day	61% 8% 16% 13% 3	1765
November	1st to 10th	Day	61% 7% 13% 14% 4 1	1487
	11th to 20th	Day	63% 8% 14% 11% 4	1520
	21st to end	Day	66% 7% 14% 12% 2	1436
December	1st to 10th	Day	68% 8% 10% 10% 4	1330
	11th to 20th	Day	60% 6% 16% 14% 3	1680
	21st to end	Day	56% 10% 14% 17% 4	2442
January	1st to 10th	Night	65% 7% 13% 12% 3	4737
	11th to 20th	Night	72% 6% 9% 11% 2	4750
	21st to end	Night	66% 7% 11% 12% 3	4960
February	1st to 10th	Night	55% 6% 13% 17% 9%	2516
	11th to 20th	Night	47% 6% 17% 21% 9%	2404
	21st to end	Night	52% 7% 12% 19% 11%	2026
March	1st to 10th	Night	53% 7% 11% 18% 10% 1	3608
	11th to 20th	Night	38% 9% 17% 24% 11% 1	3480
	21st to end	Night	50% 7% 11% 22% 10% 1	3817
April	1st to 10th	Night	41% 6% 11% 23% 16% 2	1869
	11th to 20th	Night	47% 5% 8% 21% 16% 2	1689
	21st to end	Night	47% 5% 10% 24% 12% 1	1710
May	1st to 10th	Night	45% 7% 10% 24% 12%	2634
	11th to 20th	Night	41% 8% 12% 28% 9% 3	2341
	21st to end	Night	44% 6% 11% 26% 13% 1	2414
June	1st to 10th	Night	38% 9% 17% 23% 12% 1	1330
	11th to 20th	Night	40% 8% 14% 22% 14% 2	1330
	21st to end	Night	37% 8% 19% 26% 9% 1	1330
July	1st to 10th	Night	29% 8% 21% 34% 7% 1	2185
	11th to 20th	Night	23% 7% 23% 38% 7% 1	2178
	21st to end	Night	26% 6% 23% 36% 8% 1	2337
August	1st to 10th	Night	31% 5% 16% 35% 11%	2681
	11th to 20th	Night	29% 7% 25% 31% 7%	2825
	21st to end	Night	35% 6% 21% 31% 7%	3135
September	1st to 10th	Night	32% 6% 18% 31% 12%	1826
	11th to 20th	Night	42% 7% 16% 25% 9% 1	2063
	21st to end	Night	43% 5% 14% 29% 8% 1	2090
October	1st to 10th	Night	47% 6% 16% 22% 9%	2256
	11th to 20th	Night	52% 7% 14% 23% 5%	2275
	21st to end	Night	58% 6% 14% 18% 4	2589
November	1st to 10th	Night	59% 6% 15% 14% 5% 1	2591
	11th to 20th	Night	63% 7% 13% 12% 4	2660
	21st to end	Night	67% 7% 13% 10% 3	2740
December	1st to 10th	Night	65% 7% 15% 10% 3	2850
	11th to 20th	Night	61% 7% 13% 14% 4	3516
	21st to end	Night	58% 8% 12% 15% 6%	5225

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 7% 35% 29% 13% 4	270
	11th to 20th	Day	10% 9% 27% 38% 16% 1	270
	21st to end	Day	13% 7% 32% 31% 13% 3	297
February	1st to 10th	Day	15% 8% 33% 33% 9% 2	270
	11th to 20th	Day	12% 11% 32% 27% 14% 3	270
	21st to end	Day	9% 6% 39% 33% 12%	225
March	1st to 10th	Day	10% 11% 34% 30% 14% 1	270
	11th to 20th	Day	11% 12% 31% 27% 15% 4	270
	21st to end	Day	14% 9% 30% 29% 16% 2	297
April	1st to 10th	Day	13% 9% 29% 35% 11% 3	488
	11th to 20th	Day	17% 13% 31% 26% 13% 1	640
	21st to end	Day	17% 10% 35% 26% 10% 2	650
May	1st to 10th	Day	18% 12% 32% 24% 11% 2	653
	11th to 20th	Day	21% 11% 26% 27% 13% 3	675
	21st to end	Day	22% 10% 24% 27% 14% 2	734
June	1st to 10th	Day	17% 13% 28% 24% 13% 5%	680
	11th to 20th	Day	18% 12% 33% 26% 9% 3	680
	21st to end	Day	13% 8% 26% 31% 16% 5%	680
July	1st to 10th	Day	8% 8% 30% 27% 18% 8%	671
	11th to 20th	Day	10% 9% 30% 28% 18% 4	668
	21st to end	Day	8% 6% 29% 29% 19% 8%	739
August	1st to 10th	Day	7% 9% 29% 30% 18% 6%	676
	11th to 20th	Day	11% 6% 25% 32% 18% 8%	665
	21st to end	Day	15% 9% 30% 27% 17% 3	748
September	1st to 10th	Day	11% 8% 29% 35% 15% 2	563
	11th to 20th	Day	17% 11% 38% 22% 9% 2	324
	21st to end	Day	21% 8% 36% 27% 8% 1	300
October	1st to 10th	Day	18% 15% 28% 27% 8% 4	300
	11th to 20th	Day	18% 9% 34% 31% 8% 1	297
	21st to end	Day	28% 11% 28% 21% 9% 2	297
November	1st to 10th	Day	22% 10% 28% 29% 10% 1	270
	11th to 20th	Day	14% 15% 37% 27% 7% 1	270
	21st to end	Day	16% 13% 37% 28% 6%	270
December	1st to 10th	Day	14% 8% 32% 30% 14% 3	270
	11th to 20th	Day	16% 7% 40% 27% 9% 1	270
	21st to end	Day	15% 6% 28% 32% 14% 4	297
January	1st to 10th	Night	11% 8% 33% 31% 15% 2	950
	11th to 20th	Night	16% 5% 32% 35% 11% 1	950
	21st to end	Night	14% 7% 33% 33% 12% 1	1045
February	1st to 10th	Night	13% 6% 31% 36% 14% 1	950
	11th to 20th	Night	11% 6% 31% 34% 14% 3	929
	21st to end	Night	11% 6% 34% 34% 13% 1	770
March	1st to 10th	Night	13% 6% 33% 32% 14% 1	686
	11th to 20th	Night	12% 7% 39% 29% 12% 1	570
	21st to end	Night	19% 7% 28% 29% 14% 3	627
April	1st to 10th	Night	15% 6% 29% 37% 11% 1	570
	11th to 20th	Night	18% 7% 26% 34% 13% 1	563
	21st to end	Night	16% 8% 31% 34% 11%	570
May	1st to 10th	Night	20% 6% 34% 30% 9%	570
	11th to 20th	Night	18% 7% 30% 32% 12% 1	561
	21st to end	Night	20% 7% 32% 31% 10%	617
June	1st to 10th	Night	15% 6% 29% 33% 17%	570
	11th to 20th	Night	18% 6% 31% 32% 13% 1	570
	21st to end	Night	14% 6% 20% 38% 20% 1	570
July	1st to 10th	Night	7% 4 25% 41% 22% 1	555
	11th to 20th	Night	8% 4 27% 41% 19% 1	552
	21st to end	Night	9% 3 23% 40% 22% 3	624
August	1st to 10th	Night	8% 4 22% 44% 21% 1	567
	11th to 20th	Night	9% 5% 23% 37% 23% 3	564
	21st to end	Night	15% 4 24% 38% 18% 1	627
September	1st to 10th	Night	12% 6% 29% 35% 16% 2	570
	11th to 20th	Night	18% 6% 27% 37% 12%	570
	21st to end	Night	20% 5% 32% 32% 11% 1	570
October	1st to 10th	Night	23% 7% 30% 31% 8%	736
	11th to 20th	Night	22% 6% 33% 29% 9% 1	759
	21st to end	Night	24% 9% 29% 29% 8% 1	989
November	1st to 10th	Night	20% 8% 37% 25% 9% 1	925
	11th to 20th	Night	20% 9% 32% 29% 7% 2	950
	21st to end	Night	19% 10% 40% 24% 7% 1	950
December	1st to 10th	Night	12% 7% 33% 35% 11% 2	950
	11th to 20th	Night	16% 7% 36% 31% 8% 1	950
	21st to end	Night	18% 6% 29% 31% 13% 1	1045

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